Flourescent lamps are a well known and cost effective means for providing light in both commercial and residential environments. Flourescent lamps are readily available in many sizes. However, flourescent lamps suffer from many inherent drawbacks. For example, fluorescent lamps are negative differential resistance devices, so as more current flows through them, the electrical resistance of the fluorescent lamp drops, allowing even more current to flow. Connected directly to a constant-voltage power supply, a fluorescent lamp would rapidly self-destruct due to the uncontrolled current flow. To prevent this, fluorescent lamps must use an auxiliary device, a ballast, to regulate the current flow through the tube. The simplest ballast for alternating current use is an inductor placed in series, generally having a winding on a laminated magnetic core. The inductance of this winding limits the flow of AC current. This type is still used, for example, in 120 volt operated desk lamps using relatively short lamps. Ballasts are rated for the size of lamp and power frequency. Where the mains voltage is insufficient to start long fluorescent lamps, the ballast is often a step-up autotransformer with substantial leakage inductance (so as to limit the current flow). Either form of inductive ballast may also include a capacitor for power factor correction. It follows naturally that ballasts are expensive items that must be replaced intermittently. Also, the light output and performance of fluorescent lamps is critically affected by the temperature of the bulb wall and its effect on the partial pressure of mercury vapor within the lamp. Each lamp contains a small amount of mercury which must vaporize to support the lamp current and generate light. At low temperatures the mercury is in the form of dispersed liquid droplets. As the lamp warms, more of the mercury is in vapor form. At higher temperatures, self-absorption in the vapor reduces the yield of UV and visible light. Since mercury condenses at the coolest spot in the lamp, careful design is required to maintain that spot at the optimum temperature, around 40° C. It again follows naturally that special steps should be taken for the disposal of flourescent tubes, since they cause mercury to leak into landfills and thereby cause contamination.
A light-emitting diode (LED) is a semiconductor-based light source. Many different semiconductors, such as silicon, gallium arsenide, copper indium gallium selenide, or others can be used in their manufacture. When a light-emitting diode is forward biased (switched on), electrons are able to recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence. The color of the light, corresponding to the energy of the photon, is determined by the energy gap of the particular semiconductor substrate used. LEDs are often small in area, approximately 2 mm squared, and integrated optical components may be used to shape their radiation pattern. LEDs present many advantages over flourescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, and faster switching. Efficient lighting is needed for sustainable architecture. A typical 13 watt LED lamp can emit 450 to 650 Lumens which is equivalent to a standard 40 watt incandescent bulb. LEDs have become more efficient, so that a 6 Watt LED can easily achieve the same results. The lifetime of a flourescent lamp depends on many factors, including operating voltage, manufacturing defects, exposure to voltage spikes, mechanical shock, frequency of cycling on and off, lamp orientation, and ambient operating temperature, among other factors. Typically, the lifetime is 6,000-10,000 hours. The life of a flourescent lamp is significantly shorter if it is turned on and off frequently, as is the case in residential and some commercial applications. LEDs, on the other hand, have a lifetime of 50,000 hours.
LED based lighting could be a cost-effective option for lighting a home or office space because of their very long lifetimes. Consumer use of LEDs as a replacement for conventional lighting system have been hampered in the past by the high cost and low efficiency of available products. However, there are LEDs available as efficient as 150 lm/W and even inexpensive low-end models typically exceed 50 lm/W, but are not available as drop-in replacements for flourescent lamps.